Laser-markable substrate, and associated manufacturing method

ABSTRACT

The present invention relates to a laser-markable substrate, comprising at least one material capable of forming a colored mark under laser irradiation and at least one tamper-proof agent producing an irreversible visible coloration when an attempt is made to chemically erase the mark.

The present invention relates to the field of laser-markable substrates, especially to security documents, in particular to security paper, and inter alia relates to a substrate yielding a colored mark under laser irradiation and the associated manufacturing method.

In the description, the term “paper” is understood to mean any sheet obtained by wet processing using a suspension of natural cellulose fibers and/or mineral fibers and/or plant or organic fibers other than cellulose fibers, optionally synthetic fibers, possibly containing various fillers and various additives commonly used in papermaking.

BACKGROUND

In order to produce visible marks (or inscriptions) on a substrate, especially marks such as linear or matrix (2D) barcodes, it is known to use laser marking, for example marking with an infrared laser, especially a CO₂ laser, a YAG (yttrium aluminum garnet) laser or else an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser.

The laser marking of a standard paper substrate, for example for printing writing on, or of a security paper, leads to low-contrast marks, generally having a yellow or brownish color, associated with the carbonization of the substrate during marking.

Thus, to improve the contrast of the laser marking, it is known to incorporated into or apply onto paper or plastic substrates laser-sensitive materials such as mineral fillers, inorganic pigments including conductive pigments and/or variable optical-effect pigments such as pearl-luster silicates sold especially under the name Lazerflair® (LS range) by Merck.

Patent application US 2006/0090868 describes a fibrous laser-markable substrate comprising 1 to 5% absorbent materials in the form of inorganic platelets based on pearl-luster conductive pigments.

However, although these laser-sensitive materials based on conductive pigments, or especially variable-optical-effect or interference-effect pigments, substantially improve the contrast of the marking (increasing the optical density), they do not enable the dark marking, especially the black marking, required for automatic optical recognition of marks, for example such as barcodes.

In order to provide substrates that, under laser irradiation, yield black marks, coating compositions comprising laser-sensitive materials of different nature have been developed by various companies. For example, compositions sold by Datalase under the trade name Packmark®, by Ciba under the trade name Pergamark® or else by Fujifilm Hunt Chemicals under the trade name Codestream®, are known. These compositions are coated locally onto a substrate, especially a paper, and make it possible to obtain a gray or black marking on the coated substrate after laser irradiation of the coated region. These compositions comprise for example a binder and an oxyanion of a multivalent metal, such as that provided in patent application WO 02/074548. These compositions may also be carbohydrate-based.

However, these compositions are costly to implement, especially because they must be locally applied to the substrate to be marked using an off-line method such as printing. Furthermore, these compositions are not and cannot be adapted to an on-line implementation, especially during manufacture of the paper, for example for the following reasons:

-   -   their formulations are chemically incompatible with the binders         usually used in the manufacture and conversion of paper,         especially for the facing of the paper, for example using size         press or film press methods;     -   their ability to withstand high temperatures, for example the         temperatures used during the manufacture of plastic substrates         or in the drying devices commonly used in the papermaking field,         is poor (this is the case for example of laser-sensitive         compositions based on carbohydrates such as saccharose or         sucrose, or based on encapsulated leuco dyes); and     -   they modify the properties of the surface of the paper,         especially its printability, for example offset printability;         this is the case of compositions comprising a high proportion of         fillers.

Patent application DE 102 32 786 describes a laser-markable material that makes it possible to obtain a marking in relief that provides a tactile effect.

One of the main advantages of the laser marking disclosed in the prior-art documents is its ability to resist tampering, whether mechanical or chemical. Patent applications WO 99/16625, WO 98/48398, EP 0 308 904 and FR 2 496 936 provide in particular tamper-resistant substrates comprising laser-markable materials. In particular, due to bonding, during irradiation (marking), between the laser-markable material and the substrate, and during anchoring of the laser-markable material in the substrate, the markings created on said substrates are supposed to be tamper-resistant, in particular mechanically and chemically indelible.

The applicant has however observed that it is possible to chemically remove the markings obtained by laser irradiation on substrates comprising laser-sensitive materials, or coated locally with compositions sensitive to laser marking, in particular such as described above, especially using certain chemicals or solvents such as oxidizing agents or strong bases for example. Thus, substrates comprising these laser-sensitive materials or coated with these laser-sensitive compositions, and marked by laser irradiation, do not have the properties required to thwart attempts to chemically tamper with the laser marking on these substrates. These properties are however vital to any security application, and especially in the field of security documents.

In particular it is relatively easy to remove a laser marking with chemicals such as sodium hypochlorite (bleach) or sodium hydroxide (caustic soda), inter alia.

SUMMARY

There is consequently a need to obviate at least some of the aforementioned drawbacks.

There is in particular a need to provide a laser-markable substrate, i.e. a substrate yielding a colored mark under laser irradiation, having a composition that is optimized so as to enable on-line incorporation, in particular in a paper machine, of laser-sensitive materials.

There is furthermore a need to obtain a laser-markable substrate having printing properties, especially offset printing properties, substantially analogous to those of a traditional paper substrate.

In view of the applicant's observations described above, and despite the teachings of the prior art, there is also a need to obtain a laser-markable substrate that makes it possible to detect any attempt to tamper with it, in particular chemically, by attempting to remove the laser markings.

The invention aims to meet all or some of these needs.

One subject of the invention, according to one of its aspects, is thus a laser-markable substrate comprising at least one material able under laser irradiation to form a colored mark, and at least one antitampering agent producing a visible coloration, especially an irreversible visible coloration, when an attempt is made to chemically remove the mark, making it possible to detect said removal attempt.

The term “irreversible” is intended to mean “permanent”, i.e. said coloration cannot be removed and does not disappear with time.

The mark and/or the coloration may for example be visible, especially to the naked eye or using an optical magnifying device, whether under normal (visible), infrared (IR) and/or ultraviolet (UV) light.

The mark and/or the coloration may for example form a spot or blotch, characters, for example such as an ideogram or alphanumeric signs, a symbol, an image, an acronym, a word or a code.

By virtue of the invention it is possible to ascertain whether there has been an attempt to remove the mark by dint of the presence of a colored product obtained by chemical reaction between said at least one anti-chemical-tampering agent, present in or on the substrate, and the chemicals used by the fraudsters in their attempt to remove the marks produced beforehand, by laser irradiation on the substrate. Said colored product forms a spot or blotch on the substrate.

The antitampering agent may for example be incorporated into or on the substrate during its manufacture.

It may for example be incorporated:

into the thickness of the substrate, for example by mixing it into the fibrous suspension in the case of a paper substrate or into the plastic material intended to be extruded or injection-molded in the case of a plastic substrate;

by impregnation, for example using a size press method;

by facing, for example using a film press method;

by coating, using a method such as air knife coating, roll coating, curtain coating, or bar coating; or else

by spraying.

The antitampering agent may be incorporated on-line or off-line. Preferably it is incorporated on-line during manufacture of the substrate.

The antitampering agent may for example be incorporated over the entire area of the substrate or locally, for example in a continuous band or in a particular pattern.

The antitampering agent may for example make it possible to prevent chemical removal of the mark, produced by laser irradiation of the substrate, using at least one of the following compounds:

-   -   an oxidizing agent;     -   a strong or weak base;     -   a strong or weak acid; or     -   a polar or nonpolar solvent.

The antitampering agent may make it possible to prevent chemical removal of the mark, produced by laser irradiation of the substrate, using oxidizing agents, for example such as sodium hypochlorite. In this case the antitampering agent may be chosen from the following compounds:

-   -   derivatives of manganese, nickel, cobalt or cerium and in         particular their salts;     -   pp-diaminodiphenyl, its sulfate derivatives or its phosphate         derivatives;     -   vanadium derivatives;     -   4,4′-diaminodinaphthyl (1,1′);     -   S,N-orthophenylene isothiourea;     -   primuline;     -   guanidine derivatives such as diarylguanidine derivatives, such         as for example diphenylguanidine; or     -   derivatives of thiazole, benzothiazole or aminonaphthothiazole.

The antitampering agent may make it possible to prevent chemical removal of the mark, produced by laser irradiation of the substrate, using strong bases, for example such as caustic soda, and/or weak bases such as for example sodium carbonate. In this case the antitampering agent may be chosen from the following compounds:

derivatives of manganese, nickel, cobalt or cerium and especially their salts;

colored indicators such as para-nitrophenol, phenolphthalein, pyranine or a xanthene derivative.

The antitampering agent may make it possible to prevent chemical removal of the mark, produced by laser irradiation of the substrate, using strong acids, for example such as sulfuric acid, and/or weak acids such as for example oxalic acid. In this case the antitampering agent may be chosen from the following compounds:

-   -   a colored indicator such as a xanthene derivative;     -   ferric chloride added to potassium ferrocyanide; or     -   a thiazole derivative, benzothiazole or aminonaphthothiazole.

The antitampering agent may make it possible to prevent chemical removal of the mark, produced by laser irradiation of the substrate, using polar solvents, for example such as ethanol, and/or nonpolar solvents such as for example toluene. In this case the antitampering agent may be chosen from dyes that are insoluble in water but soluble in oils and solvents. These dyes are especially used in the dyeing of petroleum derivatives.

The antitampering agent may be compatible with the adhesives used for labels, protective films or stamps.

In particular, if the substrate is an adhesive substrate, the antitampering agent of said substrate is compatible with the adhesive of said adhesive substrate.

According to another particular case, the antitampering agent is compatible with the adhesive of a label, a protective film, especially protecting the information-containing pages of a passport, or a stamp intended to make contact with or be applied to the substrate. This is because certain adhesives used may react with certain antitampering agents. These may be in particular certain antitampering agents that make it possible to prevent chemical removal using a nonpolar solvent. A person skilled in the art may therefore find it necessary to select one or more antitampering agents so as to avoid an undesired reaction with said adhesive of a label, a protective film or a stamp intended to make contact with or be applied to said substrate, and in particular it may be necessary to avoid certain antitampering agents that make it possible to prevent chemical removal using a nonpolar solvent.

The substrate may for example comprise a single antitampering agent making it possible to prevent chemical removal using a plurality of removal products. It may for example comprise manganese sulfate, and preferably manganese chloride, so as to make it possible to prevent removal of the mark using bases, for example such as caustic soda, but also oxidizing agents such as sodium hypochlorite, and in both cases the appearance of a brown spot or blotch indicates the attempted removal.

The substrate may also comprise a plurality of antitampering agents making it possible to prevent, respectively, chemical removal using a plurality of removal products, especially causing colors to appear indicative of each removal product. It may for example comprise a thiazole derivative and a xanthene derivative so as to make it possible to prevent, respectively, chemical removal using oxidizing agents such as sodium hypochlorite, via the appearance especially of an orange spot or blotch indicative of a removal attempt, but also chemical removal using bases such as caustic soda, via the appearance especially of a blue spot or blotch indicative of a removal attempt, and finally chemical removal using acids such as sulfuric acid, via the appearance especially of a pink spot or blotch indicative of a removal attempt.

The invention may make it possible for a user to easily notice any attempt, using a chemical, to remove marks produced by laser irradiation on the substrate, especially by causing a color change to appear on the substrate in the location where the chemical was applied.

The choice of material able to form a colored mark under laser irradiation may depend on the contrast required and the wavelength of the laser irradiation used. In particular, the commercially available product Micabs from Merck is a material that reacts to a marking laser that emits light at a wavelength between 100 nm and 11.5 μm. A material that reacts to infrared marking lasers, especially CO₂ marking lasers, emitting light at a wavelength between 9 and 11.5 μm, may in particular be chosen from the following commercially available products: Mark-it® from Engelhard, Packmark® from Datalase, Fast Mark® from Polyone, CerMark® from DMC2 or Lazerflair® from Merck.

The material able to form a colored mark under laser irradiation may furthermore cause a relief to be formed, especially by swelling under the effect of said laser irradiation.

The laser used to create a mark may be:

-   -   a gas laser, especially a neutral atomic laser, for example a         helium/cadmium laser emitting in the UV/visible at a wavelength         of 442 nm, an ionized atomic laser or a molecular laser, for         example a nitrogen (N₂) laser emitting in the UV at a wavelength         of 337 nm or a CO₂ laser emitting at a wavelength lying between         9 and 11.5 μm;     -   a solid-state laser, for example a ruby laser, a neodymium-doped         glass laser, an alexandrite laser or a YAG laser, optionally a         rare-earth-doped YAG laser;     -   a liquid laser (or a dye laser), for example emitting in the UV         at a wavelength lying between 100 and 350 nm or for example a         laser pumped by an X₂ YAG laser or by an N₂ laser; or     -   a semiconductor laser, for example a gallium arsenide laser.

Preferably, the material able to form a colored mark under laser irradiation will be chosen from derivatives of mica, or else from transition metals that provide a color change via a change of oxidation state, such as molybdates, tungstates and vanadates, and in particular octamolybdates, heptamolybdates and amine molybdates. Such materials are especially described in international patent application WO 02/074548.

The material able to form a colored mark under laser irradiation may for example be incorporated into or on the substrate during its manufacture.

It may for example be incorporated:

-   -   into the thickness of the substrate, for example by mixing it         into the fibrous suspension in the case of a paper substrate or         into the plastic material intended to be extruded or         injection-molded in the case of a plastic substrate;     -   by impregnation, for example using a size press method;     -   by facing, for example using a film press method;     -   by coating, using a method such as air knife coating, roll         coating, curtain coating, or bar coating; or else     -   by spraying.

The material may be incorporated on-line or off-line. Preferably it is incorporated on-line during manufacture of the substrate.

Preferably, the material able to form the colored mark will be incorporated into the facing composition of the substrate, especially a paper substrate.

The material may for example be incorporated over the entire area of the substrate or locally, for example in a continuous band or in a particular pattern. If required, said continuous band or said particular pattern may especially be respectively superposed on or covered by said continuous band or said particular pattern in which the antitampering agent has been incorporated into the substrate.

The material may be able to be marked by a laser such as those described above, especially a UV laser or an infrared laser, in particular a CO₂ and/or a YAG laser, inter alia. The material may also be able to be marked by a femtosecond laser.

The laser-markable material and the antitampering agent may be mixed into a single composition, especially into an ink, a lacquer or varnish, a coating composition, a facing composition, or an impregnating composition. Said composition may be incorporated using the methods described above, over the entire area of the substrate or locally, for example in a continuous band or in a particular pattern.

The substrate may also comprise mineral paper fillers such as for example kaolin, talc, titanium dioxide, aluminum silicate, alumina hydrate, calcined kaolin, inter alia. The mineral paper fillers may be introduced into the thickness of the substrate so as to increase the opacity and improve the printability, especially the offset printability, of the substrate.

The choice of the mineral paper fillers used in the substrate may also influence the contrast of the marking obtained.

In particular, it is preferable to use a mineral filler that is absorbent in the infrared so as to improve the contrast obtained by the laser marking. The applicant has demonstrated that, of the commonly used mineral paper fillers, alumina trihydrate and above all calcined kaolin increase the contrast of the marking produced, in particular using a CO₂ laser. It is for example possible to use calcined kaolin sold under the trade name Ansilex 93® by Engelhard.

The substrate may also comprise surface binders. The presence of certain surface binders may also make it possible to improve the contrast.

The surface binders may be chosen for example from polyvinyl alcohol (PVA), starch or a latex, alone or in a mixture. These surface binders may be accompanied or not accompanied by insolubilizers and surface bonding products so as to provide the substrate with better final printability, especially offset printability.

Preferably, the surface binders may be formed of PVA, especially highly hydrolyzed PVA, for example more than 94% hydrolyzed, thereby providing a high contrast, for example to markings made using a CO₂ laser.

The substrate may be a plastic substrate.

The substrate may be made of paper.

The substrate may for example be based on cellulose fibers (in particular cotton fibers) and/or natural organic fibers other than cellulose fibers and/or synthetic fibers, for example such as polyester fibers or polyamide fibers and/or optionally mineral fibers, for example such as glass fibers.

The substrate may comprise one or more prints, for example produced by offset printing, intaglio printing, laser printing, inkjet printing, photogravure, screen printing and/or flexography, inter alia.

The concentration of material able to form a colored mark may lie between 0.1 and 10% by dry weight relative to the weight of the substrate, and preferably between 1 and 10%.

The concentration of the antitampering agent that reacts during an attempt to chemically remove the mark may lie, if necessary for each antitampering agent used, between 0.01 and 5% by dry weight relative to the weight of the substrate.

The substrate may comprise on each of its sides an amount of laser-sensitive material lying between 0.1 and 3 g/m² by dry weight.

The substrate may have a grammage lying between 30 and 200 g/m².

The substrate may comprise a conventional ink or a security ink, especially fluorescent, phosphorescent, magnetic, photochromic, thermochromic or piezochromic ink, inter alia.

The substrate may comprise an authenticating and/or identifying security element.

Certain security elements can be detected with the naked eye, in visible light, without using a particular apparatus. These security elements comprise for example a watermark, colored fibers or platelets, printed, metallized or holographic threads, holographic foils, or variable-optical-effect prints.

These security elements are called first-level security elements.

Other types of security element can only be detected using a relatively simple apparatus such as a lamp emitting in the ultraviolet or infrared. These security elements for example comprise fibers, platelets, strips, threads or particles. These security elements may or may not be visible to the naked eye, being for example luminescent under a Wood's lamp emitting at a wavelength of 365 nm.

These security elements are called second-level security elements.

Detection of yet further types of security element requires more sophisticated detection apparatus. These security elements are for example capable of generating a specific signal when they are subjected, whether simultaneously or not, to one or more external sources of excitation. The automatic detection of the signal makes it possible to authenticate, if required, the document.

These security elements for example comprise tracers that take the form of an active material, particles or fibers, capable of generating a specific signal when these tracers are subjected to optronic, electrical, magnetic or electromagnetic excitation.

These security elements are called third-level security elements.

The substrate may comprise at least one first-, second- and/or third-level security element.

Another subject of the invention, according to another of its aspects, is the use of a laser-markable substrate, as defined above, in a security document and/or a security article.

The security document may for example be a payment means, such as a banknote, a check, a purchase order or a meal ticket, an identity document such as an identity card or an official document such as a visa, a passport or driver's license, a lottery ticket, a travel ticket or else a ticket for entry to cultural or sporting events.

The security article may for example be chosen from a security label, packaging, especially packaging for medicinal products, foodstuffs, cosmetics, perfumes, electronic parts or spare parts, a sheet used in the field of medicine or in hospitals, especially paper used to produce sterile packaging, or else a type of paper used for art.

Yet another subject of the invention, according to another of its aspects, is a method for manufacturing a laser-markable substrate such as defined above.

The method may comprise a step consisting of introducing, by impregnation, especially using a size press method, or by facing, especially using a film press method, during manufacture of the substrate in a paper machine, the material able to form a colored mark under subsequent laser irradiation.

EXAMPLES Preliminary remark

The substrates described in the following examples were marked with a CO₂ laser having the following properties:

-   -   laser wavelength 10600 nm;

×2 beam expander (beam size increased by a factor of two);

-   -   focal distance 125 nm;     -   127 nm lens; and     -   speed of 3500 mm/s.

The marked papers were then evaluated in terms of the contrast of the laser marking using an optical-density measurement and in terms of the ability of the marking to withstand chemical removal attempts using acid, caustic soda, sodium hypochlorite, ethanol and toluene.

Formulation 1 (Reference Sample)

A laser-markable substrate was manufactured using a conventional paper machine and using the following formulation in accordance with the prior art.

Pulper (refining to 35 degrees Schopper-Riegler (°SR))

-   -   long fibers: 60%; and     -   short fibers: 40%.

Mixer

-   -   mineral filler: 15% by dry weight of calcined kaolin.

Head Box

-   -   retention agent: 1.5% by weight, relative to the weight of the         paper produced in the paper machine, of cationic starch, for         example based on potatoes; and     -   bonding agent: 3.5% by weight, relative to the weight of the         paper produced in the paper machine, of a commercially available         solution of rosin-based cationic emulsion.

Size Press (Facing, for 2000 L)

-   -   binder: 200 kg of modified maize starch;     -   binder: 20 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of Lazerflair® 800 sold by         Merck.

The results showed that a yellow-brown contrast and not a black contrast was obtained by CO₂ laser marking of the paper manufactured according to formulation 1. It was observed that it was easy to remove said laser marking using sodium hypochlorite (12% chlorine) or a 5% sodium hydroxide solution.

Formulation 2

A laser-markable substrate was manufactured, using a conventional paper machine, with the same formulation as above but replacing the Lazerflair® 800 with ammonium octamolybdate.

The results showed that this time a gray-black contrast was obtained by CO₂ laser marking of the paper manufactured according to formulation 2. It was noted that it was easy to remove said laser marking using sodium hypochlorite (12% chlorine) or using a 5% sodium hydroxide solution.

Formulations 3, 4, 5, 6, 7 and 8

A laser-markable substrate was manufactured, using a conventional paper machine, with the following formulation:

Pulper (Refining to 35 Degrees Schopper-Riegler (°SR))

-   -   long fibers: 60%; and     -   short fibers: 40%.

Mixer

-   -   mineral fillers: 15% by dry weight.

Head Box

-   -   retention agent: 1.5% by weight, relative to the weight of the         paper produced in the paper machine, of cationic starch, for         example based on potatoes; and     -   bonding agent: 3.5% by weight, relative to the weight of the         paper produced in the paper machine, of a commercially available         solution of rosin-based cationic emulsion.

Size Press (Facing, for 2000 L)

-   -   binder: 200 kg of modified maize starch;     -   binder: 20 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of ammonium octamolybdate.

Tests were carried out using various mineral paper fillers other than kaolin (used in formulation 2): rutile titanium dioxide (formulation 3), precipitated calcium carbonate (formulation 4), sodium aluminosilicate (formulation 5), aluminum hydroxide (formulation 6), talc (formulation 7) and calcined kaolin (formulation 8).

The results of the CO₂-laser-marking tests (measurement of the optical density) were the following:

Formulation 4 Formulation 3 with with rutile precipitated Formulation 5 Formulation 2 titanium calcium with sodium with kaolin dioxide carbonate aluminosilicate 0.02 0.02 0.03 0.13 Formulation 6 Formulation 8 with aluminum Formulation 7 with calcined hydroxide with talc kaolin 0.10 0.08 0.23

The results showed that a higher contrast was obtained for the laser marking when calcined kaolin, sodium aluminosilicate or aluminum hydroxide were used as the mineral paper filler.

However, it was then easy to remove the laser mark using sodium hypochlorite (12% chlorine) or a 5% sodium hydroxide solution.

Formulations 9, 10, 11 and 12

Four laser-markable substrates were manufactured, using a conventional paper machine, with the bulk formulation (before facing) as follows:

Pulper (Refining to 35 Degrees Schopper-Riegler (°SR))

-   -   long fibers: 60%; and     -   short fibers: 40%.

Mixer

-   -   mineral filler: 15% by dry weight of calcined kaolin.

Head Box

-   -   retention agent: 1.5% by weight, relative to the weight of the         paper produced in the paper machine, of cationic starch, for         example based on potatoes; and     -   bonding agent: 3.5% by weight, relative to the weight of the         paper produced in the paper machine, of a commercially available         solution of rosin-based cationic emulsion.

Next, on-line, on the above fibrous mat, the four following size press formulations are applied respectively:

Size Press (Facing, for 2000 L) for Formulation 9

-   -   binder: 200 kg of modified maize starch;     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of ammonium octamolybdate.

Size Press (Facing, for 2000 L) for Formulation 10

-   -   binder: 200 kg of modified maize starch;     -   binder: 20 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of ammonium octamolybdate.

Size Press (Facing, for 2000 L) for Formulation 11

-   -   binder: 100 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of ammonium octamolybdate.

Size Press (Facing, for 2000 L) for Formulation 12

-   -   binder: 100 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   surface bonding agent: 50 kg of a modified acrylic styrene         derivative;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution; and     -   laser-sensitive material: 250 kg of ammonium octamolybdate.

Better results were obtained in terms of the contrast of the CO₂ laser marking on the paper manufactured according to formulations 11 and 12 when a surface binder in the form of polyvinyl alcohol (PVA) was used.

However, it was then easy to remove the laser mark using bleach (12% chlorine) or a 5% sodium hydroxide solution.

Moreover, the paper manufactured according to formulation 12 had a better offset, laser and inkjet printability than that manufactured according to formulation 11.

Formulation 13

A laser-markable paper substrate was manufactured according to the invention using a conventional paper machine with the following properties:

Pulper (Refining to 35 Degrees Schopper-Riegler (°SR))

-   -   long fibers: 60%; and     -   short fibers: 40%.

Mixer

-   -   mineral filler: 15% by dry weight of calcined kaolin; and     -   antitampering agent based on xanthene (colored indicator): 0.3%         by weight of the commercially available solution relative to the         weight of the paper produced in the paper machine.

Head Box

-   -   retention agent: 1.5% by weight, relative to the weight of the         paper produced in the paper machine, of cationic starch, for         example based on potatoes; and     -   bonding agent: 3.5% by weight, relative to the weight of the         paper produced in the paper machine, of the commercially         available solution of rosin-based cationic emulsion.

Size Press (Facing, for 2000 L)

-   -   binder: 100 kg of polyvinyl alcohol (PVA);     -   antifoam: 0.35 kg of an aqueous emulsion of an aliphatic mixture         of hydroxylated compounds;     -   insolublizing agent: 3 kg of a commercially available         modified-glyoxal solution;     -   laser-sensitive material: 250 kg of ammonium octamolybdate; and     -   antitampering agent: 20 kg of manganese chloride.

A paper having a high contrast, especially to CO₂ laser markings, was obtained. In addition an attempt to remove the laser marking, for example using sodium hypochlorite (12% chlorine) or a 5% sodium hydroxide solution, led to a brown colored spot or blotch appearing, indicating that an attempt had been made to tamper with the marking on the paper. An attempt to remove the laser marking using an acid, such as for example 5% sulfuric acid solution, led to the appearance of a pink colored spot or blotch that also indicated an attempt had been made to tamper with the marking on the substrate.

The expression “comprising a” is intended to be synonymous with “comprising at least one”. 

1. A laser-markable substrate, comprising: at least one material able to form a colored mark under laser irradiation; and at least one antitampering agent producing an irreversible visible coloration when an attempt is made to chemically remove the colored mark.
 2. The substrate as claimed in claim 1, the antitampering agent being incorporated into the thickness of the substrate by impregnation, facing, coating or spraying.
 3. The substrate as claimed claim 1, the antitampering agent making it possible to prevent chemical removal of the mark containing at least one of the following compounds: an oxidizing agent; a strong or weak base; a strong or weak acid; or a polar or nonpolar solvent.
 4. The substrate as claimed in claim 1, the antitampering agent making it possible to prevent chemical removal of the mark using oxidizing agents and being chosen from: derivatives of manganese, nickel, cobalt or cerium; pp-diaminodiphenyl, its sulfate derivatives or its phosphate derivatives; vanadium derivatives; 4,4′-diaminodinaphthyl (1,1′); S,N-orthophenylene isothiourea; primuline; guanidine derivatives; or derivatives of thiazole, benzothiazole or aminonaphthothiazole.
 5. The substrate as claimed in claim 1, the antitampering agent making it possible to prevent chemical removal using strong and/or weak bases and being chosen from: derivatives of manganese, nickel, cobalt or cerium; colored indicators.
 6. The substrate as claimed in claim 1, the antitampering agent making it possible to prevent chemical removal of the mark using weak and/or strong acids and being chosen from: a colored indicator; ferric chloride added to potassium ferrocyanide; or a thiazole derivative, benzothiazole or aminonaphthothiazole.
 7. The substrate as claimed in claim 1, the antitampering agent making it possible to prevent chemical removal with polar and/or nonpolar solvents and being chosen from dyes that are insoluble in water but soluble in oils and solvents.
 8. The substrate as claimed in claim 1, the antitampering agent being a manganese derivative.
 9. The substrate as claimed in claim 1, the at least one material able to form a colored mark under laser irradiation being incorporated into the thickness of the substrate by impregnation, facing, coating or spraying.
 10. The substrate as claimed in claim 1, the at least one material being able to be marked by a UV laser or by an infrared laser, in particular a CO2 laser and/or a YAG laser.
 11. The substrate as claimed in claim 1, said colored mark having a relief.
 12. The substrate as claimed in claim 1, the at least one material able to form a mark being chosen from derivatives of mica, molybdates, tungstates and vanadates.
 13. The substrate as claimed in claim 1, further comprising a mineral filler absorbent in the infrared.
 14. The substrate as claimed in claim 1, further comprising polyvinyl alcohol (PVA).
 15. The substrate as claimed in claim 1, wherein the substrate is based on cellulose fibers.
 16. The substrate as claimed in claim 1, wherein the substrate is a plastic substrate.
 17. The substrate as claimed in claim 1, further comprising at least one authenticating and/or identifying security element.
 18. The substrate as claimed in claim 1, wherein the concentration of material able to form a colored mark lying between 0.1 and 10% by dry weight relative to the weight of the substrate.
 19. The substrate as claimed in claim 1, wherein the concentration of the at least one antitampering agent lying between 0.01 and 5% by dry weight relative to the weight of the substrate.
 20. A security document and/or a security article, comprising: a laser-markable substrate, comprising: at least one material able to form a colored mark under laser irradiation, and at least one antitampering agent producing an irreversible visible coloration when an attempt is made to chemically remove the colored mark.
 21. A method of manufacturing a laser-markable substrate as claimed in claim 1, comprising: introducing into the thickness of the substrate, by impregnation or by facing during manufacture of the substrate in a paper machine, material of the substrate able to form a colored mark under laser irradiation. 